Even when the complement cascade fails to proceed beyond generating the C3 convertase, complement activation is effective at inducing pathogen uptake and destruction. This process of immune protection is mediated by:

Correct Answer: C

Rationale: C3b and fragments (iC3b) engage phagocyte complement receptors for uptake ; not inhibitory , proteases , or B cells .

Correct Answer: C

Rationale: Closed circulatory systems are named so because blood remains confined within vessels, unlike open systems where it bathes tissues directly.

Correct Answer: C

Rationale: Vaccines often use weakened or inactivated pathogens to stimulate immunity, not antibodies, B cells, or antibiotics.

Correct Answer: B

Rationale: HIV targets helper T cells, not mast cells (involved in allergies), making B false; others are true.

Correct Answer: A

Rationale: NK cells, of lymphoid origin, target altered host cells (e.g., virus-infected or cancerous), not bacteria or antigens directly.